I'm trying to get a WxC fax-to-email number working properly. I've had the WxC people fax me for testing and I received a TIFF file by e-mail so it appears that its working. But when trying to send a fax to the number from a friend's place (he has a WxC VFX Line, Grandstream HT-01 Single Port ATA), I can't seem to send faxes to my fax-to-email number.

When using on-hook dialling w/ the fax machine, I hear WxC pick up the call but then WxC's end hangs up almost immediately and then I hear a 'busy' tone. After a few attempts, the fax machine gives up and prints out a report stating the fax was unable to be sent because the line was "BUSY".

I can however send & receive to/from other numbers so my mate's fax machine would appear to be functional.

Where can I start with figuring out why this isn't working? TIA.